Plan to kill Farallones mice a balancing act

On the craggy, guano-covered Farallon Islands, tiny brown blurs of fur dart furtively across the ground between thousands of holes.

There is precedent for using brodifacoum. It was used on Anacapa Island off the coast of Santa Barbara, Calif. by the National Parks Service in 2002. While it resulted in some collateral damage, the poison successfully removed non-native black rats, which were damaging a small bird called the Xantus's Murrelet. That species has since made a comeback, according to studies.

In 2008 dozens of bald eagles and hundreds of gulls were killed when the poison was used to rid Rat Island in the Aleutians of its namesake species.

No matter what decision is made, any plan would have to kill all of the tens of thousands of mice to be effective, a difficult proposition.

"All it takes is one pregnant female to survive. With the mouse population estimated at 40,000, getting every single mouse to consume the poison pellets will not happen," said Maggie Sergio, director of advocacy for WildCare, a group opposed to the use of mouse poison.

"So the question needs to be asked; if you cannot guarantee every single rodent will be eradicated and we know there will be a devastating impact to other species and the environment, why do this?"

Pete Warzybok, one of five Point Reyes Bird Observatory biologists currently living in an old house on the island, said the fate of the Storm-petrel and other species is a good reason why. Studies have shown a 40 percent decline in Storm-petrels on the Farallones over the past 20-years, and a corresponding increase in burrowing owls.

Warzybok said factors such as climate change and other predators are also at play in the Storm-petrel's decline, but that eradicating mice is a concrete step that can help that and other species survive.